Protection of molybdenum and tungsten at high temperatures



Nov. 27, 1956 PROTECTION OF MOLYBDENUM AND TUNGSTEN AT HIGH TEMPERATURES M. F. QUAELY ETAL 2,772,227

Filed June 29, 1953 CHROMl UM-VANADIUMQOR CHROMlUM-NlCKEL-VANADIUM COMPLEX 3 VINVENTORS MARTIN F. QUAELY WM. C. LILLIENDAHL A TTORN United SwesPatdi i PROTECTION OF MOLYBDENUM m STEN AT men TEMPERATURES Martin F. Quaely, Montclair, and William c. Lilliendahl,

' This invention relatesjto readilyoxidiaable metalsiand improved means for protecting them. against oxidation at elevated temperatures. 1 The principal object of our invention, generallyronsidered,.is to produce ,articlesfrom refractory readilyoxidizable metals in which -oxidation thereof at elevated temperatures is prevented by acomplex coating, which is not only firmly adherent, but flexible without peeling from the base material. U

,-Another object of our invention is to protect articles which are constructed of refractory but readily-oxidizable metals, such as molybdenum, tungsten, alloys thereof with eachother, and alloys thereof with smallproportions of metal of the group consisting of cobalt, nickel and iron, comprisingcleaning said articles,.and electroplating them with a black chromium-vanadium or chromium-nickelvanadium complex. o o

A further object of our invention is the protection of molybdenum and other materials having similar oxidation characteristics with coatings of improved ductility and resistance consisting ofblack complex chromium base coatings electrolytically applied and including a proportion of, vanadium.

Other objects and advantages of the invention will becomeapparent as the description proceeds.

Referring to the drawing:

Figure 1 is a yertical ,sectional viewof a plating bath which may be usedfor coating articles in accordance with our invention.

Figure 2 is a transverse sectional view of;an article of molybdenum coated or plated with a chromium-vanadium rapidly formed but is volatile, so that at any temperaturethereabove it not only oxidizes whenin contact with an oxidizing atmosphere, but the formed oxide volatilizes very quickly, and at a correspondingrate as the temperature is increased. It is obvious that under such conditions molybdenum is very soon destroyed unless protected.

Various methods forprotecting molybdenum, tungsten, I alloys thereof with each other, and alloys thereof having.

approximately the oxidation properties of .such metals, such as those with only small proportions of metal 'of the group consisting of cobalt, nickel and iron, have been proposed and tried. By the expression alloys of molyb denum with small proportions of metal selected from the group consisting of cobalt, nickel, iron a nd tungsten,

we mean-such in Whichthe proportion ofany one of the 3 is a vertical sectional view of a furnace in 2,772,227 Patented, Noya .27, 1956 first three mentioned, alloyingmetalsis not greater than 15%, and in which the proportion of tungsten is not greater than 20%. In the same way the expression alloys of tungsten with small proportions of metal selected from the group consisting of cobalt, nickel, iron and molybdenum, means such in which the proportion of any one ofthe first three mentioned alloying metals is not greater than and in which the proportion of molybdenum is not greater than 20%., It will, however, be

understood that our invention is also suitable for protect- At the present time, probably the most widely used protection for molybdenum is a coating of molybdenum silicide. Such av coating may be produced by vapor depositing a thin layer of silicon on the hot surface of molybdenum. ,It can also be formedby a painted or sprayed on and sintered method, or one such as described and claimed in the Garrison and Lovett application, Ser. No. 759,256, filed July 5, 1947, now patent No. 2,650,903, filed July 5, 1947, and owned by the assignee of the present application. While such a coating has very good protective value at elevated temperatures, its brittleness may result in cracking when struck or deformed. In ordertobe of value, any-protective coating must completely cover the molybdenum or tungsten surface. Any defects, such as a crack or pinhole makes the coating useless since molybdenum, or other similar material, will erode by oxidation at the pointof defect.

Other suggested methods of protection for molybdenum are the formation of alloys thereon, the application of ceramic coatings, and cladding. All of such methods however are very limited in application, and there is still a great need for protective coatings for molybdenum and the like.

In accordance with our invention, we employ electrodeposited coatings for the protection of metals of the class contemplated. Specifically, we propose a process whereby a black complex chromium base coating is applied electrolytically to the surface of the metal to be protected. Such a coating is preferably a chromiumvanadium or chromium-nickel-vanadium complex.

Before applying such a coating to a surface to be protected against oxidation, the same is thoroughly cleaned to remove any oil, grease, or contamination therefrom. This may be accomplished by methods now in general use for such purpose. However, a more thorough cleaning can be given, in addition to the above, by firing the molybdenum in a non-carburizing reducing atmosphere, such as is provided by hydrogen. Such a treatment produces a very or tank 11 of material resistant to the action of the.

electrolyte, such as glass or other ceramic, is shown holding an electroplating bath 12 of a composition consisting of an aqueous solution of chromium trioxide desirably prepared from chromium trioxide (C103)- and a vanadium saltor salts, in the ratio of about 20 parts-chromium-to aboutlpart vanadium. Although any soluble vanadium salt, not including harmful mater1al, may be employed, we prefer. ammonium metavanardate; However, ,other. soluble vanadium compounds,v such as vanadium halide or the nitrate, may be sub stituted.

- Atypical solution contains chromium trioxide (CrOs), ZOO-grams per liter, ammonium metavanadate, 20 grams per-liter, and glacialflacetic acid, 6.5 ml. per liter.. The acetic acid, or. other soluble carboxylic acid, such formic, citric, oxalic or propionic' acid is addedin a proportion of from 3 to 20grams per literto increase the working temperature range and obtain more uniform coatings. I I

Another-electrolyte that can be used to produce such evident lo'ss 'of"molybdenum. The higher the tempera-' fired in air at 1200 C. for several hours without any ture of the firing in hydrogen and the longer the duration of this firing, the more diffusion of the coating into the molybdenum occurs. The use to which the article is to be put determines .whethenit is permissibly fired at such, ,temperatures., I I

Althoughgpreferred.fembodiments have been disclosed,

' it will be understood that modifications may be made protectivecoatings contains chromium trioxide, nickel chloride, and a soluble vanadium salt, in an approximate ratio of parts chromium to 4 parts nickel to 1 part vanadium. A typical solution' contains chromium trioxide '(Cros), 200 grams per liter; nickel chloride (NiC'l2-6H2O), 80 grams per liter; and 5 grams per liter of vanadium dissolved in just sufiicient nitric acid toeffect solution of the metal. To each liter of' solution, 5 to 10ml. of glacial acetic acid, or other soluble carboxylic acid, is added. Nickel sulfate is not a subwithin. the, spirit: and scope .of.i.the. invention;

We claim: 1 a r 1. The method'of protecting against oxidation-at high temperatures articles constructed of metal-ofthe group consistingpfmolybdenum; tungsten, and alloys thereof with each otherand with small proportions of metal of the group consisting of cobalt, 'nickel*and iron, comprising cleaning said articles, and electroplating by making them cathodes in an electrolytic cell with lead anodes,

and 'in' which the electrolyte consists of an aqueoussolutionofi 'chronioacid "containing 150 to 350"'grams of chromium trioxide per liter and vanadium salt containinglto 10 grams-of vanadium per liter, with a ratio .of about twenty'parts chromium to one part'vanadium,

stitute' for nickel chloride in such an electrolyte, as the sulfate radical produces inferior coatings.

After electroplating from one ofthe solutions before mentioned, a-black uniform deposit is obtained upon the-metal surface with a current-density of one ampere per square centimetenand at a potential difference between the electrodes 13 and 14 of about l2'to 15 volts. The temperature of the bath is desirably maintained between'35" and 50 C., cooling coils (not shown) being Care The application of several layers insures comdepends on the length of time the plating is continued.

A desirable thickness is from .001" to .005", which may be obtained in'a period of from 5 to 25 minutes. I

After plating, the oxidizable refractory metal has a protective coating on its surface, which is not only resistant to oxidation when heated in an oxidizing atmos phere, but it is also resistant to the action of nitric acid and many other chemical reagents, except hydrochloric acid by which it is slowly-attacked.

Durability of the coating may be improved by firing in-hydrogen'forseveral hours at about 1200 C. An electric furnace,-for this purpose, is illustrated at '15 inmFig. 3. In such a furnace the plated molybdenum or-the like is-heated in a streamof hydrogen, or other non-carburizing reducing or protective atmosphere in which hydrogen is the active ingredient, as'by means of an electrical resistance element 16 surrounding a tube 17 ofalumina to about 1200 C. for preferably several hours. furnace in a boat 19, of alumina or other inert refractory material, while subjected to the'action of the reducing atmosphere.

After the firing in hydrogen -or the like, additional layers of the coating can be applied, followed by another cycle of firing in the reducing atmosphere after the application of each layer. In thisway the' coating is at, least partially diffused into the metal to be protected,

thereby increasing its protectiveproperties and bonding,

and a dense, protectiveHcoating is obtained. Molyb-v enum. 1 x mP e,-... oated. n..-.thi.s m nner has :been

The'coatedmaterial 13 may be held insaidsaidele'ctrolyte'also-containing acetic acid to increase the working temperaturerange and obtain more uniform coatings, With a current density during plating of about= one ampere per square centimeter, with a potential dif-' .fereneeofabout 12 to 15 volts, and a bath temperature of-between35- and-509 C., to' 'produce thereon a chro mium-vanadium complex.

2. The=method= of protecting against oxidation at high temperatures articles constructed of-metal of the groupconsisting'ofmolybdenum, tungsten, and alloys thereof wit-h each other and'with smalbproportions of metal of the group consisting of cobalt, nickel and iron, compris-- ing cleaningsaid'articles, electroplating 'by'making them cathodes inkanelectrolytic 'cell' with lead anodes, and

in which"the 'electrolyte consists'of an aqueous solution of chromic-acidcontaining 150 to'350 grams ofchromium trioxide per liter-and vanadium'salt' containing 2 to 10 grams of vanadium per liter, with a ratio of about twenty parts'chromiumto one part vanadium, said elecing temperature range and obtain more uniform coatings,-'with-a-current' density during plating of about one ampere per square centimeter, with a potential difference of about 12 to 15 volts, and a bath temperature of between *35? and C., to produce thereon a chr0- .trolyte also containing acetic-acid 'to'increase 'thework mium-vanadium' complex, and firing in hydrogen for several hours at about 1200 C. to diffuse the coatings into the articles.

3. The method of protecting against oxidation at high.

temperatures articles constructed of "metal'of the group consisting -of"inolybdenum, tungsten, and alloys thereof wi th*eachother-andwithsmallproportions of metal of the group consisting of, cobalt, nickel and iron, comprising cleaning said, articles, and electroplatingby making themca'thodes in an electrolytic cell in which the electroly't e consists, of an aqueous solution of chromic acid containing to '350 grams of chromium trioxide. per liter, and vanadium salt containing 2 to 10 grams of vanadium per; liter, said electrolyte also containing acetic acid to increase ;the working temperature range I,

temperature articles constructed of metal of the group,

consisting ,of molybdenum, tungsten, and. alloys thereof.

with each otherand with small proportions of. metalof the group consisting of;cobalt, nickel and iron, comprising A leaning said garticles, electroplatingby making 7 them}- cathodes in an electrolytic cell in which the electrolyte consists of an aqueous solution of chromic acid containing 150 to 350 grams of chromium trioxide per liter and vanadium salt containing 2 to 10 grams of vanadium per liter, said electrolyte also containing acetic acid to increase the working temperature range and obtain more uniform coatings, with a current density during plating of about one ampere per square centimeter, and a bath temperature of between 35 and 50 C., to produce thereon a chromium-vanadium complex, and firing in hydrogen for several hours to diifuse the coatings into the articles.

5. The method of protecting against oxidation at high temperatures articles constructed of metal of the group consisting of molybdenum, tungsten, and alloys thereof with each other and with small proportions of metal of the group consisting of cobalt, nickel and iron, comprising cleaning said articles, and electroplating by making them cathodes in an electrolytic cell in which the electrolyte consists of an aqueous solution of chromic acid containing about 200 grams of chromium trioxide References Cited in the file of this patent UNITED STATES PATENTS 1,565,724 Fonda Dec. 15, 1925 1,590,101 Kyropoulos June 22, 1926 1,795,459 Westbrook Mar. 10, 1931 1,975,239 Ungelenk et a1. Oct. 2, 1934 2,144,250 Allen et a1. Jan. 17, 1939 2,304,297 Anton Dec. 8, 1942 2,512,141 Ma et a1. June 20, 1950 2,555,372 Ramage June 5, 1951 2,623,847 Gilbert Dec. 30, 1952 

4. THE METHOD OF PROTECTING AGAINST OXIDATION AT HIGH TEMPERATURE ARTICLES CONSTRUCTED OF METAL OF THE GROUP CONSISTING OF MOLYBDENUM, TUNGSTEN, AND ALLOYS THEREOF WITH EACH OTHER AND WITHSMALL PROPORTIONS OF METAL OF THE GROUP CONSISTING OF COBALT, NICKEL AND IRON, COMPRISING CLEANING SAID ARTICLES, ELECTROPLATING BY MAKING THEM CATHODES IN AN ELECTROLYTIC CELL IN WHICH THE ELECTROLYTE CONSISTS OF AN AQUEOUS SOLUTION OF CHROMIC ACID CONTAINING 150 TO 350 GRAMS OF CHROMIUM TRIOXIDE PER LITER AND VANADIUM SALT CONTAINING 2 TO 10 GRAMS OF VANADIUM PER LITER,SAID ELECTROLYTE ALSO CONTAINING ACETIC ACID TO 